Ideally, when a lamp is powered from a sine wave source, such as the electrical grid, it is desirable for the lamp to appear to the source as purely resistive. That is, it should draw current from the source in phase with, and closely matching, the voltage waveform. This minimizes stress on the electricity generating and transmission infrastructure. However, as input voltage amplitude varies, the light output from the lamp should remain fairly constant. This requires line regulation circuitry.
While line regulation seeks to maintain constant light output as the voltage applied to the lamp decreases, dimmers are intended to reduce lamp brightness by effectively reducing voltage supplied to the lamp. When an external dimmer reduces supply voltage, line regulation circuitry in the lamp responds by increasing input current in order to maintain relatively constant power and lamp brightness, thus rendering the dimmer ineffective.
Of greater concern is that at low dimming levels, line regulation circuitry can increase input current to destructive levels, which is a safety issue.
Complex circuitry is often required to detect the presence of a dimmer and even more circuitry to respond appropriately.
Driving LEDs from the AC line requires unique qualities. While the AC line is a constant-voltage sine wave, LEDs require constant DC current to operate. There exists many ways to drive a matrix of LEDs from an AC source. Conventionally, an inductive approach is taken, using an inductive element to store and release energy at different input and output voltages. This is the traditional switching power supply approach. This has the disadvantages of utilizing bulky, expensive magnetics, generating EMI due to the high switching frequencies involved, a difficulty in achieving good power factor, the use of limited-lifetime electrolytic capacitors, requiring high current devices, and difficulties with dimmer compatibility.
To address these problems and to simplify circuitry, multi-stage, sequentially-operated linear regulators have been used to drive a segmented string of LEDs. Instantaneous input current roughly follows input voltage, providing low distortion and good power factor. And they are inherently compatible with most dimmers.
As depicted in FIG. 1, in a prior art, multi-stage, sequentially-operated linear regulator, a long string of LEDs is tapped at multiple locations, each tap having a linear current regulator that controls the LED current of upstream string segments. As instantaneous rectified input voltage rises, downstream segments get enough voltage to begin conducting. Once a segment starts conducting, the downstream regulator takes over from the upstream regulator. This limits the voltage across the regulators when they are conducting, minimizing power loss.
Typically, each regulator operates at a fixed current level. Thus the input current waveshape is a stairstep rather than a smooth sine wave, causing input current harmonics.
Line regulation has also been a problem in the prior art. That is, output power, and thus brightness, varies as time-averaged input voltage varies.
What is needed is an improved current control circuit for an LED driver that overcomes these problems and shortcomings in the prior art.